upgrader: by default, we should remove useless business template

It is possible to add script Base_getUpgradeBusinessTemplateList to define
the list of business template you would like to keep if needed

bt5/erp5_upgrader/SkinTemplateItem/portal_skins/erp5_upgrader/TemplateTool_checkBusinessTemplateInstallation.xml

@@ -54,6 +54,7 @@
 \n
 template_tool = context.getPortalObject().portal_templates\n
 return template_tool.upgradeSite(bt5_list,\n
+  delete_orphaned=True,\n
   keep_bt5_id_set=keep_bt5_id_set,\n
   dry_run=(not fixit))\n
 </string> </value>